The need to reduce the weight of automotive vehicles and to increase safety has led to the production of high-strength steels.
Historically, the development of these types of steel began with the addition of alloy elements to primarily achieve precipitation hardening.
Later, “dual phase” steels were proposed which contain martensite in a ferrite matrix to obtain structural hardening.
To obtain superior strength levels combined with good deformation properties, “TRIP” (Transformation Induced Plasticity) steels have been developed, the microstructure of which contains a ferrite matrix containing bainite and residual austenite which, under the effect of a deformation such during a stamping operation, for example, is transformed into martensite.
Finally, to achieve mechanical strength greater than 800 MPa, multi-phase steels with a majority bainite structure have been proposed. These steels are used in industry, and in particular in the automobile industry, to manufacture structural components.
This type of steel is described in EP 2 020 451. To obtain an elongation at failure greater than 10% as well as mechanical strength greater than 800 MPa, the steels described in this publication, in addition to the known presence of carbon, contain manganese and silicon, molybdenum and vanadium. The microstructure of these steels essentially contains upper bainite (at least 80%) as well as lower bainite, martensite and residual austenite.
However, the fabrication of these steels is expensive on account of the presence of molybdenum and vanadium.